Double sided heat exchanger cooling unit

ABSTRACT

A cooling unit positioned between a first and a second gas stream, the first and the second gas stream having no direct fluid contact therebetween. The cooling unit includes a double-sided heat exchanger with a first side that is in thermal communication with the first gas stream and a second side that is in thermal communication with the second gas stream. The double-sided heat exchanger provides a direct path of thermal conduction between the first gas stream and the second gas stream. First fins are provided on the first side of the double-sided heat exchanger and second fins are provided on the second side of the double-sided heat exchanger. A first surface area of the first side of the double-sided heat exchanger is at least 5% greater than a second surface area of the second side of the double-sided heat exchanger. A housing surrounds a fan and the second fins.

BACKGROUND OF THE INVENTION

Electrical enclosures are used to house and protect electronicsequipment from potentially harmful environments such as high humidity orrain, condensation, solar heat loads, dust and debris, temperatureextremes and damaging corrosion. In some cases, the cabinets must besealed to a standard, typically by National Electrical ManufacturersAssociation (NEMA). For example, a NEMA 12 cabinet is for indoor use andprotects against drips, dust and falling dirt. A NEMA 4 cabinet is forindoor or outdoor use and protects against the same things as NEMA 12 aswell as hose directed spray. The sealed nature of these cabinetsrequires heat transfer from the inside space to the ambient environmentwhile maintaining the NEMA rated seal.

For most electronic components in an enclosure, the suggested maximumallowable air temperature within the enclosure is in the range of 130°F. to 160° F. One common guideline states that for every 18° F. thetemperature of an electronic component is elevated beyond itsrecommended operating temperature, the lifetime of the electroniccomponents is cut in half. In order to increase electrical componentlifetime and reduce process downtime due to component failure orreplacement, a cooling solution must be chosen. If an enclosure must besealed from the ambient environment, then a method of transferring heatthrough some cooling device must be chosen based on the amount of heatthat needs to be removed and the ambient conditions around theenclosure.

In cases where the ambient air temperature is suitably low, anair-to-air heat exchanger 10, as shown diagrammatically in FIG. 1, canbe used to transfer heat from the air 12 inside of the cabinet 14 to theambient air 16. An air-to-air heat exchanger 10 transfers heat from thehot air 12 inside the enclosure 14 to the cooler ambient air 16 whilemaintaining a NEMA seal that prevents external air and othercontaminants from entering the enclosure. It is beneficial to use a heatexchanger instead of an air conditioner (AC) when possible because theenergy required to operate the cooling system is much lower.

Two key factors influencing the amount of heat that can be transferredfrom air inside of an enclosure to ambient air are the surface areaavailable for heat transfer and the thermal resistance through thecooling device. Increasing the surface area of a heat exchanger byadding an extended surface or fin structure can increase heat transferthrough the heat exchanger.

Lowering the thermal resistance between the internal enclosure air andthe external ambient air reduces the difference between the two airtemperatures that is necessary to transfer a given amount of heat. Thismeans that with a lower thermal resistance, a lower internal enclosuretemperature can be maintained for a given heat load into the enclosure.Air-to-liquid coolers are used for enclosures with high heat loads.These systems transfer heat to a liquid supply that may be a coolingwater supply or a chilled liquid loop. In either case there is arequirement for additional equipment or a water supply that may notalways be available.

Thermoelectric coolers are commonly used to achieve sub-ambienttemperatures within an enclosure, but they are limited in capacity andare very expensive relative to other types of cooling products.

Air conditioners are also used to achieve sub-ambient cooling and arethe most common active enclosure cooling product. The drawback tocompressor-based cooling systems is that they require greater energyinput than air-to-air heat exchangers.

When outside air cannot be introduced to the interior of an enclosurebut the maximum temperature of the ambient environment and maximum heatload in the enclosure are suitable, a heat exchanger can be used totransfer heat out of the enclosure while maintaining a seal between theambient environment and the interior of the enclosure. Heat exchangerproducts that are currently in use include heat pipe heat exchangerswith aluminum finned coils, folded-fin heat exchangers, and plate & finheat exchangers.

SUMMARY

An object of the invention is to provide a double-sided heat exchangerthat has a direct path of thermal conduction between a first gas streamon one side of the double-sided heat exchanger and a second gas streamon the other side of the double-sided heat exchanger.

An object of the invention is to provide a heat exchanger with a lowerthermal resistance by providing heat pipes in the heat exchanger. Theheat pipes can be embedded between an internal side of a double-sidedheat exchanger and the external side, so that heat can be transferredwith little thermal resistance to the external side.

An object of the invention is to provide a heat exchanger in whichembedded heat pipes ensure uniform temperature distribution across theheat exchanger, allowing for maximum heat transfer efficiency.

An object of the invention is to provide a heat exchanger in whichembedded heat pipes may be used to transfer heat from an internal heatsink to an external heat sink that is larger in size. This adds heattransfer area at the exterior surface where heat must be dissipated toambient air. If the heat transfer characteristics of the external heatsink are improved, then less air flow may be required to dissipate theheat.

An embodiment is directed to a cooling unit positioned between a firstgas stream and a second gas stream, the first gas stream and the secondgas stream having no direct fluid contact therebetween, the cooling unitincluding a double-sided heat exchanger having a first side having afirst planar surface area, the first side with a first heat sink that isin thermal communication with the first gas stream, the first sidefacing the first gas stream, and a second side having a second planarsurface area, the second side with a second heat sink that is in thermalcommunication with the second gas stream, the second side facing thesecond gas stream, the double-sided heat exchanger providing a directpath of thermal conduction between the first gas stream and the secondgas stream, the first planar surface area of the first side of thedouble-sided heat exchanger is at least 25% greater than the secondplanar surface area of the second side of the double-sided heatexchanger. The cooling unit further includes the first heat sinkincluding a first conductive plate and first fins, and the second heatsink including a second conductive plate and second fins, the firstconductive plate having a length greater than a corresponding length ofthe second conductive plate. The cooling unit further includes heatpipes provided between the first conductive plate and the secondconductive plate, the heat pipes extending along the length of the firstconductive plate. The cooling unit further includes the first finsextending in a direction away from the first conductive plate of thefirst heat sink of the double-sided heat exchanger, and the second finsextending in a direction away from the second conductive plate of thesecond heat sink of the double-sided heat exchanger, the first fins andthe second fins increase the heat transfer between the first gas streamand the second gas stream. The cooling unit further includes a fansurrounded by and directly connected to a housing surrounding the secondfins and directly attached to the second conductive plate. The coolingunit further includes the double-sided heat exchanger is made from athermally conductive material.

Another embodiment is directed to a cooling unit positioned between afirst gas stream and a second gas stream, the first gas stream and thesecond gas stream having no direct fluid contact therebetween, thecooling unit including a double-sided heat exchanger having a first sidehaving a first planar surface area, the first side with a first heatsink that is in thermal communication with the first gas stream, thefirst side facing the first gas stream, and a second side having asecond planar surface area, the second side with a second heat sink thatis in thermal communication with the second gas stream, the second sidefacing the second gas stream, the double-sided heat exchanger providinga direct path of thermal conduction between the first gas stream and thesecond gas stream, the first planar surface area of the first side ofthe double-sided heat exchanger is at least 25% greater than the secondplanar surface area of the second side of the double-sided heatexchanger. The cooling unit further includes the first heat sinkcomprising a first conductive plate and first fins, and the second heatsink comprising a second conductive plate and second fins, the firstconductive plate having a length greater than a corresponding length ofthe second conductive plate. The cooling unit further includes the firstfins extending in a direction away from the first conductive plate ofthe first heat sink of the double-sided heat exchanger, and the secondfins extending in a direction away from the second conductive plate ofthe second heat sink of the double-sided heat exchange, the first finsand the second fins increase the heat transfer between the first gasstream and the second gas stream. The cooling unit further includes afan surrounded by and directly connected to a housing surrounding thesecond fins and directly attached to the second conductive plate. Thecooling unit further includes a high conductivity layer is providedbetween the first conductive plate of the first heat sink of thedouble-sided heat exchanger and the second conductive plate of thesecond heat sink of the double-sided heat exchanger, the highconductivity layer enhances the thermal conduction of the double-sidedheat exchanger.

Other features and advantages of the present invention will be apparentfrom the following more detailed description, taken in conjunction withthe accompanying drawings which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generic air-to-air heat exchanger diagram of the prior art.

FIG. 2 is a top perspective view of an illustrative heat exchanger ofthe present invention for use in a cabinet.

FIG. 3 is a diagrammatic side view of the heat exchanger of FIG. 2positioned in a cabinet, with the cabinet shown in phantom to betterillustrate the positioning of the heat exchanger.

FIG. 4 is a front perspective view of an alternate illustrative heatexchanger of the present invention for use in a cabinet, the heatexchanger having a fan provided thereon.

FIG. 5 is a back perspective view of the heat exchanger of FIG. 4.

FIG. 6 is an exploded perspective view of the heat exchanger of FIG. 4.

FIG. 7 is a portion of a representative heat exchanger formed as asingle block, the heat exchanger having openings for heat pipes.

FIG. 8 is a portion of a representative heat exchanger formed from twoblocks which are joined together, the heat exchanger having openings forheat pipes formed by the two blocks.

FIG. 9 is a diagrammatic view of an air-to-air heat exchanger accordingto the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The description of illustrative embodiments according to principles ofthe present invention is intended to be read in connection with theaccompanying drawings, which are to be considered part of the entirewritten description. In the description of embodiments of the inventiondisclosed herein, any reference to direction or orientation is merelyintended for convenience of description and is not intended in any wayto limit the scope of the present invention. Relative terms such as“lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,”“down,” “top” and “bottom” as well as derivative thereof (e.g.,“horizontally,” “downwardly,” “upwardly,” etc.) should be construed torefer to the orientation as then described or as shown in the drawingunder discussion. These relative terms are for convenience ofdescription only and do not require that the apparatus be constructed oroperated in a particular orientation unless explicitly indicated assuch. Terms such as “attached,” “affixed,” “connected,” “coupled,”“interconnected,” and similar refer to a relationship wherein structuresare secured or attached to one another either directly or indirectlythrough intervening structures, as well as both movable or rigidattachments or relationships, unless expressly described otherwise.Moreover, the features and benefits of the invention are illustrated byreference to the preferred embodiments. Accordingly, the inventionexpressly should not be limited to such preferred embodimentsillustrating some possible non-limiting combination of features that mayexist alone or in other combinations of features; the scope of theinvention being defined by the claims appended hereto.

In general, as represented diagrammatically in FIG. 9, an electricalenclosure or cabinet 102 includes a cooling unit with a double-sidedheat exchanger 100 positioned between a first gas stream 104 and asecond gas stream 106 is disclosed. The double-sided heat exchanger 100provides a direct path of thermal conduction between the first gasstream 104 and the second gas stream 106.

Referring to FIG. 3, a double-sided heat exchanger 100 positioned in anelectrical enclosure 102. The heat exchanger 100 is positioned between afirst gas stream 104 provided inside the enclosure 102 and a second gasstream 106 positioned outside the enclosure 102. The first gas stream104 and the second gas stream 106 have no direct fluid contacttherebetween. The double-sided heat exchanger 100 is positioned in awall 108 of the enclosure 102 with a first side 110 with a first heatsink 111 in thermal communication with the first gas stream 104 and asecond side 112 with a second heat sink 113 in thermal communicationwith the second gas stream 106. The double-sided heat exchanger 100provides a direct path of thermal conduction between the first gasstream 104 and the second gas stream 106. In the embodiment shown inFIG. 2, the cooling unit has first fins 114 provided on the first heatsink 111 of the first side 110 of the double-sided heat exchanger 110and second fins 116 provided on the second heat sink 113 of the secondside 112 of the double-sided heat exchanger 100. Referring to FIG. 2,the fins 114, 116 increase the heat transfer between the first gasstream 104 and the second gas stream 106. However, other embodiments ofthe heat exchanger, which are not shown, may have fins only on one heatsink, or may have no fins on either heat sink. The double-sided heatexchanger 100 is made from a thermally conductive material. Thethermally conductive material may include, but is not limited to,aluminum, aluminum alloy, copper, copper alloy or stainless steel.

Embedded heat pipes 118 are provided between the first heat sink 111 andthe second heat sink 113. The heat pipes 118 extend essentially theentire length or width of either the first heat sink 111 or the secondheat sink 113, whichever is longer.

In the embodiment shown, the second gas stream 106 is a colder fluidstream located outside of an enclosure 102 which houses the double-sidedheat exchanger 100, and the first gas stream 104 is a warmer fluidstream located inside of the enclosure 102 which houses the double-sidedheat exchanger 100. In various embodiments, a surface area of the secondside 112 of the double-sided heat exchanger 100 is larger than thesurface area of the first side 110 of the double-sided heat exchanger100. The surface area of the second side 112 may be 5% greater, 10%greater, 15% greater, 20% greater, 25% greater or greater than 25%greater than the surface area of the first side 110. In addition, thesurface area of the second fins 116 may be 5% greater, 10% greater, 15%greater, 20% greater, 25% greater or greater than 25% greater than thesurface area of the first fins 114. In various embodiments, the secondfins 116 of the second side 112 are spaced apart by a distance which isgreater than a distance by which the first fins 114 of the first side110 are spaced apart, therein facilitating thermal conduction to occurby natural convection. The second fins 116 may be spaced apart by 5%greater, 10% greater, 15% greater, 20% greater, 25% greater or greaterthan 25% greater than the first fins 114. In addition, the second fins116 may be thicker than the first fins 114.

As best shown in FIG. 6, heat pipes 118 are embedded between a highlyconductive layer or plate 120 and a highly conductive layer or plate122. The highly conductive layers or plates 120, 122 may be made from,but are not limited to, aluminum, aluminum alloys, copper or copperalloys. Alternatively, the plates may be integrally molded with orextruded in the heat sinks 111, 113.

Heat pipes 118 may be replaced by a highly conductive layer positionedbetween the plates 120, 122. The highly conductive layer may be madefrom, but not limited to, pyrolytic graphite, diamond and graphite fiberreinforced composite.

In the embodiment shown in FIGS. 2 and 3, the plate 122 extends thelength of the heat sink 113 and the plate 120 extends the length of theheat sink 111, thereby allowing portions of the heat pipes 118 to beexposed. In contrast, plates 120 and 122 of FIGS. 4 through 6 aresimilar in length, thereby covering the entire length of the heat pipes118. The plates 120, 122 and heat pipes 118 are placed in good thermalcontact with the heat sinks 111, 113 to facilitate the heat transferbetween the first gas stream 104 and the second gas stream 106. The heatpipes 118 are used to facilitate the movement of heat from the heatinput zone and spread heat throughout the entire heat exchanger 100while delivering it to the heat output zone.

In various embodiments, a longitudinal axis of the one or more heatpipes 118 is parallel with a longitudinal axis of the first fins 114 andthe second fins 116. In other embodiments, the longitudinal axis of theone or more heat pipes 118 is perpendicular with the longitudinal axisof the first fins 114 and/or the second fins 116.

One or more variable conductance heat pipes can be used in place oftraditional heat pipes to allow the thermal resistance of the entireheat exchanger 100 to be increased when the temperature local to theheat output zone is reduced (such as in a low temperature ambientcondition).

The width of the heat sinks 111, 113 may be essentially identical (asshown in FIGS. 7 and 8) or they may vary (as shown in FIGS. 2 through6), with the heat sink 113 on one side 112 of the double-sided heatexchanger 100 larger than the heat sink 111 on the other side 110. Inthe case where the two sides of the double-sided heat exchanger 100 aresymmetric, the heat pipes may not be needed. In the case where one sideof the double-sided heat exchanger 100 is larger, the heat pipes 118will extend the length of the larger heat sink 113 in order to spreadheat to the entire surface of the larger heat sink 113. A larger heatsink 113 positioned outside of the enclosure 102 is useful forsituations that require natural convection heat dissipation on the heatoutput side.

As best shown in FIGS. 4 and 6, fan assemblies 130 with one or more fans132 enclosed therein can be mounted on the heat sinks 111. In addition,fan assemblies (not shown) may also be mounted on heat sinks 113. Thefan 132 facilitates the movement of air through the fins to allow heatto be dissipated by forced convection. In the embodiment shown in FIGS.4 through 6, only the heat sink 111 has an internal fan assembly 130provided thereon. This forces hot air from the first gas stream 104 ofthe enclosure 102 over the fins 114 of the heat sink 111, cooling theair. Depending on the application, one or more external fans may also bemounted on the heat sink 113 to facilitate the cooling of the heat sink113. Both internal fans and external fans can be turned off or notoperated when natural convention is sufficient to dissipate the heat.

In FIG. 6, the fan 132 is mounted in an impingement configuration sothat air is drawn into the center of the fins 114 and exhausted fromeach end. The fan assembly 130 may include a shroud 134 or ductwork thatdirects air flow in a manner that enhances heat transfer. Otherembodiments of the fan assembly 130 and fan 132 may be used withoutdeparting from the scope of the invention. For example, a low power fanmay be used to enhance natural convection heat transfer on one side.

Referring to FIG. 7, an alternate embodiment is shown. In thisembodiment, the double-sided heat exchanger 100 is formed as a singlemember or block 150, with the first fins 114 of the first heat sink 111and the second fins 116 of the second heat sink 113 formed therein. Oneor more openings 152 extend through an integral layer or plate 120formed between the first fins 114 and the second fins 116. The openings152 are dimensioned to receive heat pipes 118 therein. The heat pipes118 are used to move heat from the heat input zone and spread heatthroughout the entire heat exchanger 100 while delivering it to the heatoutput zone.

Referring to FIG. 8, another alternate embodiment is shown. In thisembodiment, the double-sided heat exchanger 100 is formed from twomembers or blocks 250, 252, with the first fins 114 of the first heatsink 111 formed in the first block 250 and the second fins 116 of thesecond heat sink 113 formed in the second block 252. Rounded surfaces254, 256 are formed on mating sides of the respective blocks 250, 252.The rounded surfaces form one or more openings 258 when the blocks 250,252 are mated or joined together. The openings 258 are dimensioned toreceive heat pipes 118 therein. The heat pipes 118 are used to move heatfrom the heat input zone and spread heat throughout the entire heatexchanger 100 while delivering it to the heat output zone.

The double-sided heat sink exchanger and the enclosure described hereinare able to be produced at lower cost than other types of coolers suchas heat pipe heat exchangers or thermoelectric coolers.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the spirit and scope of theinvention as defined in the accompanying claims. In particular, it willbe clear to those skilled in the art that the present invention may beembodied in other specific forms, structures, arrangements, proportions,sizes, and with other elements, materials, and components, withoutdeparting from the spirit or essential characteristics thereof. Oneskilled in the art will appreciate that the invention may be used withmany modifications of structure, arrangement, proportions, sizes,materials, and components and otherwise, used in the practice of theinvention, which are particularly adapted to specific environments andoperative requirements without departing from the principles of thepresent invention. The presently disclosed embodiments are therefore tobe considered in all respects as illustrative and not restrictive, thescope of the invention being defined by the appended claims, and notlimited to the foregoing description or embodiments.

The invention claimed is:
 1. A cooling unit positioned between a firstgas stream and a second gas stream, the first gas stream and the secondgas stream having no direct fluid contact therebetween, the cooling unitcomprising: a double-sided heat exchanger having a first side having afirst planar surface area, the first side with a first heat sink that isin thermal communication with the first gas stream, the first sidefacing the first gas stream, and a second side having a second planarsurface area, the second side with a second heat sink that is in thermalcommunication with the second gas stream, the second side facing thesecond gas stream, the double-sided heat exchanger providing a directpath of thermal conduction between the first gas stream and the secondgas stream, the first planar surface area of the first side of thedouble-sided heat exchanger is at least 25% greater than the secondplanar surface area of the second side of the double-sided heatexchanger; the first heat sink comprising a first conductive plate andfirst fins, and the second heat sink comprising a second conductiveplate and second fins; heat pipes provided between the first conductiveplate and the second conductive plate, the heat pipes extending alongthe length of the first conductive plate; the first fins extending in adirection away from the first conductive plate of the first heat sink ofthe double-sided heat exchanger, and the second fins extending in adirection away from the second conductive plate of the second heat sinkof the double-sided heat exchanger, the first fins and the second finsincrease the heat transfer between the first gas stream and the secondgas stream; a fan surrounded by and directly connected to a housingsurrounding the second fins and directly attached to the secondconductive plate; the double-sided heat exchanger is made from athermally conductive material.
 2. The cooling unit as recited in claim1, wherein a first fin surface area of the first fins of the first sideof the double-sided heat exchanger is at least 5% greater than a secondfin surface area of the second fins of the second side of thedouble-sided heat exchanger.
 3. The cooling unit as recited in claim 1,wherein the thermally conductive material is aluminum, aluminum alloy,copper, copper alloy or stainless steel.
 4. The cooling unit as recitedin claim 1, wherein at least one heat pipe of the heat pipes has alength having a longitudinal axis parallel with a longitudinal axis ofthe first fins and the second fins.
 5. The cooling unit as recited inclaim 1, wherein a longitudinal axis of at least one heat pipe of theheat pipes is perpendicular with a longitudinal axis of the first finsand the second fins.
 6. The cooling unit as recited in claim 1, whereinthe first gas stream is a colder fluid stream located outside of anenclosure which houses the double-sided heat exchanger and the secondgas stream is a warmer fluid stream located inside of the enclosurewhich houses the double-sided heat exchanger.
 7. The cooling unit asrecited in claim 6, wherein at least one heat pipe of the heat pipes isa variable conductance heat pipe embedded in the double-sided heatexchanger to reduce the heat transfer when the temperature of the firstgas stream drops.
 8. The cooling unit as recited in claim 1, wherein thefirst gas stream is a colder fluid stream located outside of anenclosure which houses the double-sided heat exchanger and the secondgas stream is a warmer fluid stream located inside of the enclosurewhich houses the double-sided heat exchanger, the first fins are spacedapart by a first distance which is at least 10% greater than a seconddistance by which the second fins are spaced apart, therein allowing theheat transfer to occur by natural convection.
 9. The cooling unit asrecited in claim 1, wherein the external fan is turned off when naturalconvection is sufficient to dissipate the heat.
 10. A cooling unitpositioned between a first gas stream and a second gas stream, the firstgas stream and the second gas stream having no direct fluid contacttherebetween, the cooling unit comprising: a double-sided heat exchangerhaving a first side having a first planar surface area, the first sidewith a first heat sink that is in thermal communication with the firstgas stream, the first side facing the first gas stream, and a secondside having a second planar surface area, the second side with a secondheat sink that is in thermal communication with the second gas stream,the second side facing the second gas stream, the double-sided heatexchanger providing a direct path of thermal conduction between thefirst gas stream and the second gas stream, the first planar surfacearea of the first side of the double-sided heat exchanger is at least25% greater than the second planar surface area of the second side ofthe double-sided heat exchanger; the first heat sink comprising a firstconductive plate and first fins, and the second heat sink comprising asecond conductive plate and second fins; the first fins extending in adirection away from the first conductive plate of the first heat sink ofthe double-sided heat exchanger, and the second fins extending in adirection away from the second conductive plate of the second heat sinkof the double-sided heat exchange, the first fins and the second finsincrease the heat transfer between the first gas stream and the secondgas stream; a fan surrounded by and directly connected to a housingsurrounding the second fins and directly attached to the secondconductive plate; a high conductivity layer is provided between thefirst conductive plate of the first heat sink of the double-sided heatexchanger and the second conductive plate of the second heat sink of thedouble-sided heat exchanger, the high conductivity layer enhances thethermal conduction of the double-sided heat exchanger.
 11. The coolingunit as recited in claim 10, wherein the double-sided heat exchanger ismade from a thermally conductive material, the thermally conductivematerial is aluminum, aluminum alloy, copper, copper alloy or stainlesssteel.
 12. The cooling unit as recited in claim 10, wherein the highconductivity layer is made from pyrolytic graphite, diamond or graphitefiber reinforced composite.
 13. The cooling unit as recited in claim 10,wherein the high conductivity layer includes one or more heat pipes. 14.The cooling unit as recited in claim 13, wherein the one or more heatpipes has a length having a longitudinal axis parallel with alongitudinal axis of the first fins and the second fins.
 15. The coolingunit as recited in claim 13, wherein a longitudinal axis of the one ormore heat pipes is perpendicular with a longitudinal axis of the firstfins and the second fins.
 16. The cooling unit as recited in claim 10,wherein the first gas stream is a colder fluid stream located outside ofan enclosure which houses the double-sided heat exchanger and the secondgas stream is a warmer fluid stream located inside of the enclosurewhich houses the double-sided heat exchanger.
 17. The cooling unit asrecited in claim 16, wherein the high conductivity layer is made of oneor more variable conductance heat pipes embedded in the double-sidedheat exchanger to reduce the heat transfer when the temperature of thefirst gas stream drops.
 18. The cooling unit as recited in claim 16,wherein the first fins are spaced apart by a first distance which is atleast 10% greater than a second distance by which the second fins arespaced apart, therein allowing the heat transfer to occur by naturalconvection.